Polyurethane foams are generally prepared by the reaction of one or more active hydrogen-containing compounds (i.e., polyols) and one or more polyisocyanates, in the presence of a blowing agent such as water, and usually at least one reaction catalyst and foam stabilizer. The cellular polymer structure of polyurethane foam has a skeletal framework of relatively heavy strands forming an outline for the cell structure. The skeletal framework strands are connected by very thin membranes, often called windows, which form the cell walls. In open-celled foams, some of the windows are open or torn in each cell, thus forming an interconnecting network open to fluid flow (liquid or gas).
Reticulation relates to methods for removing or breaking the cell windows of polyurethane foams. Mechanical, chemical and thermal methods for reticulating foams are known. As one example, foam may be reticulated by melting the windows with a high temperature flame front or explosion, which still leaves the strand network intact. Alternatively, the cell windows may be etched away using the hydrolyzing action of water in the presence of an alkali metal hydroxide. See U.S. Pat. Nos. 3,125,542; 3,405,217; 3,423,338; 3,425,890 and 4,670,477 for descriptions of various reticulating methods for polyurethane foams.
Household cleaning sponges and mop heads most commonly are formed from cellulose. Paper pulp is the primary ingredient for cellulose sponges. The pulp is reacted with carbon disulfide to form a soluble cellulose xanthate compound. This compound is dissolved into a honey-like liquid viscose and mixed with reinforcing fibers to add strength to the pulp mixture. The cellulose is formed with a double cell structure to replicate natural sea sponges. Sodium sulfate crystals are added to the pulps, and this mixture is heated in a mold to melt the crystals. Heating regenerates the mix to pure cellulose and leaves the signature sponge holes where the crystals have melted away. Bleaching chemicals and humectants maintain the moisture level and color purity of the cellulose sponge. While the cellulose has good water absorption and wicking, it has lower wet integrity than other materials. Moreover, upon drying, the cellulose becomes hard and brittle such that it must be pre-wet before using for wiping.
Open celled polyester and polyether polyurethane foams have greater softness and flexibility than cellulose, and retain flexibility upon drying without humectants. As compared to cellulose, polyurethane foams have greater wet strength, better wet integrity and exhibit less swelling when wet. Foams also can be foamed to have a double cell structure to more resemble natural sea sponges. Generally, polyurethane foams can be produced more cheaply than cellulose. However, polyurethane foams are hydrophobic, lacking good liquid absorption and wicking characteristics, which makes them less suitable for household sponges and mop heads. Even after the polyurethane foams are post-treated with surfactants in an attempt to improve water absorption and wicking, they still do not match the performance of cellulose for these properties. Polyurethane sponges tend to leave streaks of water on a wiped surface.
Reticulated polyurethane foams have been used as components of filters. Such foams also have been suggested for use as components of household sponges, particularly for the abrasive surface presented by a reticulated foam. See, e.g., U.S. Pat. Nos. 3,857,133 and 5,640,737.
U.S. Pat. No. 6,756,416 discloses certain hydrophilic ester polyurethane foams that are chemically reticulated to improve water holding capacity and wet strength. Such foams still do not have water absorption and wicking to match that of cellulose under all use conditions.
U.S. Pat. No. 6,841,586 discloses certain hydrophilic ester polyurethane foams that are chemically reticulated to improve water holding capacity and wet strength, and are further felted (compressed under heat and pressure) to improve water absorption rates. The added costs (chemical treatment, energy and fabrication) to produce foams under this teaching to achieve the desired water absorption make such foams prohibitively expensive in comparison to cellulose.
The art still seeks polyurethane foams suitable to replace cellulose materials as liquid absorbing and wicking components of household sponges and mop heads.